This invention relates to auto-focusing cameras in which the camera lens is automatically varied depending on subject distance.
An auto-focus camera hving an automatically positioned lens is described in the parent application. In that arrangement, when an exposure cycle is initiated, a range finder system beams a burst of ultrasonic energy toward the subject such that the interval of time between transmission of the burst and receipt of an echo is a parameter linearly related to subject distance. A scale factor is then employed to permit comparison between the linearly related time and the lens/subject function of the camera lens.
For a given lens/subject function, the known parametric relationship between range time (subject distance) and the selected lens/subject function establishes a program for a pulse repetition frequency of a scaled clock which scales the range time, the clock output being accumulated in a range counter. Such program causes the pulse repetition frequency of the scaled clock to vary in accordance with the time derivative of an approximation of the lens/subject function to allow for a piece-wise linear approximation of the actual lens/subject function (taking the minimum circle of confusion into account). As a consequence, the content of the range counter at the end of the range time interval will be a number representing the desired focal position of the lens.
In one embodiment of the parent application, receipt of an echo terminates incrementing of the range counter by the scaled clock and enables a lens drive motor that is effective to drive the lens from a park position toward the desired focal position. Movement of the lens operates an auxiliary pulse generator whose output is used to increment the range counter. When this counter is filled, a focus control signal is generated which stops the lens drive motor and actuates the shutter mechanism to thereby complete the camera cycle. In the event that the lens drive motor is enabled but the lens is not moved (i.e., the auxiliary pulse generator is not producing pulses), it is desirable to terminate the lens drive and camera operation to prevent damage.
Moreover, as is well known, the lens/subject function establishes the relationship between the distance of a subject and the position of the lens on the camera at which an image of the subject at that distance is in focus on the focal plane of the camera. Such function also establishes the range of distances over which subjects can be brought into focus by moving the lens. Thus, the function determines the close-up distance at which close subjects can be brought into focus, as well as the hyperfocal distance which is the subject distance beyond which a subject will be in focus when the lens is at its hyperfocal position.
For lens assemblies having the same or similar lens/subject function, range of lens movement often varies depending upon the use for which the camera is designed. For example, a camera designed for an exceptionally broad range of indoor and outdoor photography may permit focusing in the range of 25 cm to infinity in order to provide as wide a latitude as possible, whereas a range of 75 cm to infinity is suitable for many photographic operations. On the other hand, a lens system designed solely for indoor flash photography need not allow lens movement to the hyperfocal position where the effective range of the flash illumination is limited.
Consequently, it is an object of this invention to prevent camera damage due to electrical or mechanical malfunctions which occur during the lens drive operation.
Another object is to provide an automatic range and focus system applicable to camera apparatus having different limits of lens displacement.